Interpretive Summary: Soil can be a significant source of preharvest contamination of produce by pathogens. Demand for natural pesticides such as essential oils for organic farming continues to increase. We studied the antimicrobial activity of several essential oils against Salmonella in soil. Essential oils (cinnamaldehyde and eugenol), two bio-pesticides (Ecotrol and Sporan) containing essential oils, and an organic acid (acetic acid) at 0.5 - 2.0%, were mixed with organic soil and inoculated with five different strains of Salmonella to obtain 6 log cfu/g soil. Salmonella populations were undetectable in soil treated with as low as 0.5% cinnamaldehyde. Increases in oil concentration resulted in further reduction of salmonella with all oils used in the study. Up to six log reductions in Salmonella serovars Typhimurium, Negev and Newport were found after one day when cinnamaldehyde, Ecotrol, eugenol, Sporan, or vinegar were used at 2% level. Overall, S. Negev was the most sensitive strain to oils and cinnamaldehyde was superior to other treatments in reducing E. coli O157:H7 populations in soil. The significant reduction of salmonella could greatly reduce potential contamination of fresh organic produce inadvertently contaminated by soil.

Technical Abstract:
Salmonella can survive in soil for months to years; consequently, soil can be a preharvest source of contamination of produce. Elimination of Salmonella with natural products and processes such as essential oils is important to prevent infection among consumers. Essential oils have been mainly evaluated in liquid medium and foods in which minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) are determined. However, there are no reports describing the impact of essential oils in soil, especially organic soil. We evaluated essential oils for controlling Salmonella enterica serovars in organic soil.
Two essential oils (cinnamaldehyde and eugenol), two bio-pesticides (Ecotrol and Sporan) and an organic acid (20% acetic acid ) at 0.5%, 1.0%, 1.5% and 2.0%, were mixed with organic sandy soil and inoculated with six different serovars of Salmonella enterica separately. Soils were incubated at room temperature and samples obtained at 1, 7 and 28 days were enumerated to determine survival. The bactericidal effect of cinnamaldehyde was evident at 0.5%, 1.0%, 1.5%, and 2.0% and during all times of incubation. Overall, S. Negev was the most sensitive strain to oils resulting in significant reductions compared with other strains. Increases in oil concentration resulted in further reduction of salmonella with all oils used in the study. Up to six log reductions in Salmonella serovars Typhimurium, Negev and Newport were found after one day when cinnamaldehyde, Ecotrol, eugenol, Sporan, or vinegar were used at 2% level. This study shows the potential use of essential oils to effectively reduce salmonella populations in soil. The significant reduction of salmonella could greatly reduce potential contamination of fresh organic produce inadvertently contaminated by soil.